Boat Buying for Absolute Beginners, Part III

Choose the power that's right for your boat

By David Brown

August 9, 2001

For runabouts, stern-drive propulsion packages are the most popular choice. (Photo courtesy Glastron)

Powerboats run on iron hearts: twin or single, gas or diesel, outboard, inboard or stern drive. The combination of engine and drivetrain ultimately play a major role in a boat's on-water performance. With so many options available, it can be difficult to decide which combination is best for you.

The Good News

The choices in marine power only seem endless, especially to the novice. The fact is, not every engine and drive package in say, the MerCruiser line, is offered for every boat. Far from it. Naval architects and engineers use complex formulas to determine the correct engine options for a given boat.

So you don't need to know calculus or have a super computer to make good boat-buying decisions. The number crunching, at least in terms of engines offered for the boat that piques your interest, has been done for you before you walk into a dealer's showroom. Your job is picking the best power options from those offered for a given boat that meet your specific needs.

Gasoline or Diesel?

More good news: Nuclear power isn't offered for anything a consumer can buy, solar power is rare and steam engines are out. You have two fundamental choices: gasoline or diesel.

Choosing between gas or diesel power involves more than checking the depth of your wallet. Far more important is the boat you buy (See related item, "Boat Buying for Absolute Beginners, Part I" at right) and how you plan to use it.

Although more prevalent on fishing boats, some runabouts also are offered with outboard power. (Photo courtesy Glastron)

Diesels offer outstanding durability under frequent use. In fact, many mechanics say the primary reason for diesel engine problems is lack of use. As I've heard more than one mechanic say, "Diesels like to work."

A diesel that sits idle for weeks at a time can form rust on its cylinder walls, which can cause loss of compression, which is needed to ignite the fuel. The fuel itself can even begin to grow microorganisms. Diesels that never work harder than running at harbor speeds often "coke up," meaning they build up carbon in the cylinders. That leads to problems — and expensive repairs.

Freshwater flush systems are offered on certain outboards, such as this Evinrude 150. (Photo courtesy Evinrude)

If you plan to use your boat often, spending the extra money on diesel power if it's offered for the model you want may be wise. This is especially true if you buy a heavy boat that demands lots of muscle to push it through the water. Diesels do love work — in commercial service it's not unusual for one to go 10,000 hours between major overhauls. A properly maintained diesel engine that works hard daily can outlast several gasoline power plants of similar size.

Outboards can be used in single-, twin- and even triple- or quad-engine applications. (Photo courtesy Baja Marine)

However, diesel power isn't offered for the overwhelming majority of powerboats — those from 18 to 23 feet — sold each year. Don't worry, you're not getting short-changed by having gas power as your only option. Diesel wouldn't make sense for these boats.

In addition to costing less (at least initially), gasoline engines offer maximum flexibility in drive configuration. They generally offer superior bursts of speed and throttle response, and they don't "mind" going unused for longer periods of time. Plus, gasoline is available just about everywhere, where as sometimes diesel fuel can be hard to find.

Don't be tempted to make a direct comparison of horsepower between gasoline and diesel engines. They are measured differently. The horsepower of a gasoline engine is rated at or near wide-open throttle, which may be at 4,500 rpm or more. The actual horsepower of any engine drops quickly as the rpm drops. Diesel engines are rated at continuous duty, typically in the 2,000-to 2,200-rpm range. This is why a diesel will outperform a gasoline engine when hauling heavy loads. Of course, the gas engine has the edge when it comes to quick bursts of speed.

A Closer Look at Gas Engines

A complete stern-drive package includes an engine and the drive itself, which can be seen in the right-hand third of this photograph. (Photo courtesy Mercury Racing)

With few exceptions, there are no true marine gasoline engines available to pleasure boats less than 50 feet long. The power plants available are all "marinized" versions of engines originally developed for highway vehicles. This does not mean that an engine out of a Chevrolet, Ford or Volvo is exactly the same as the marine version built on the same basic block (the main iron casting containing the crankshaft and pistons). Important changes are made to the bearings, lube oil system, cooling system and exhaust manifold when an automotive block is converted into a marine engine.

Two marinization modifications are critical: a "wet" exhaust system and non-sparking electrical parts. Both gasoline and diesel engines must have water-cooled exhaust manifolds to prevent red-hot iron from becoming a fire hazard. Marine manifolds typically have water jackets through which engine coolant flows. In addition, cooling water is injected into the stream of hot exhaust gas. This is done at an elbow designed to prevent water from being sucked back into the cylinders. Cooling the hot exhaust muffles the engine's roar and makes it possible to route the potentially lethal gas through rubber hose to an outlet fitting at the transom.

According to U.S. Coast Guard regulation, all electrical components on gasoline engines must be spark protected. The reason is to prevent explosions. Gasoline fumes are heavier than air. They collect in the bilge where a spark from a starter motor, alternator or distributor could ignite them.

A Closer Look at Diesel Engines

Large, heavy and clunky were once the adjectives used to describe marine diesels. Today's diesels are lightweight, smooth-running machines. Unfortunately, one aspect of diesel engines has not changed: they are still expensive, often costing twice as much as a gasoline engine of similar output. Cost is the primary reason why so few powerboats are equipped with diesel power. There are other considerations, however. For instance, the power curves (the horsepower developed at various speeds) of gasoline and diesel engines are different. In general, diesels are best for long hours of constant speed pushing heavy loads.

The major difference between gasoline and diesel engines is the way in which the fuel is ignited in the cylinder. Gasoline engines require a spark plug. Diesel engines use the heat created by compressing air in the cylinder to ignite an extremely fine spray of fuel. Spark ignition requires a volatile fuel, such as gasoline. Diesel fuel is less volatile.

Drive Configurations

Although not all propulsion systems are offered with either gas or diesel engines, powerboat buyers have a wide selection of ways to propel their boats. The most popular options are:

Outboard motor — An outboard engine is a self-contained package of engine, gears and propeller that attaches to the stern of the boat. Two-stroke outboards up to 300 hp are available; four-stroke outboards commonly up to 350 hp and in one case, 557 hp. Direct-injection two-stroke outboards are far more environmentally friendly than their carbureted ancestors and, in recent years, once-temperamental DI technology has become far more reliable (see our DFI Tutorial for more information).

Conventional inboard — Sometimes called "straight inboard," this configuration has power transmitted from the engine through a shaft which exits the bottom of the boat. The propeller is attached to this shaft and rotates beneath the hull, usually directly in front of the rudder.

V-drive inboard — A V-drive allows the engine to be placed in the rear of the boat. Power goes forward to the V- drive. From there, a shaft leads out of the bottom in a manner similar to a conventional inboard boat. Using a V-drive allows boat designers to create more interior space than you'd fine in a conventional inboard, which has a motorbox in the center of the cockpit.

Stern drive — Not so long ago, this drive package was called an "inboard/outboard" because this configuration combines an inboard engine with the lower unit of an outboard motor. This combination allows the use of higher horsepower engines. The lower unit can be tilted up for trailering or shallow-water operation.

Jet drive — In this system, water is sucked from beneath the boat and shot out of a nozzle in the stern to provide forward thrust. Jet drives can operate in shallow water and have no exposed propeller. Personal watercraft (PWC) are jet driven, as are a number of compact runabouts, some of which have twin engines. Jet drives have seen a rise in popularity lately; see Jet Drive Renaissance for more information.

Pod Drives — Pod drives consist of a contained drive unit which is located beneath the hull, directly below the engine. The drive unit articulates independently, eliminating the need for rudders, shafts, and other running gear. This type of drive is rapidly replacing conventional inboards on large boats, because it's much easier to handle around the docks and can deliver efficiency gains of up to 30-percent.

A Closer Look at Stern Drives

Today's stern drives come as a full package that includes the engine, gimbal housing and lower unit. The boat builder simply cuts an opening in the back of the boat and installs this package. Stern drives, which came to life in the late 1950s, took off when small boats grew too large for then-available outboards to handle. Combining inboard engines with the outboard lower unit was a logical development with several advantages:

A stern-drive boat is easier for beginners to maneuver than a conventional inboard craft.

Adjusting the angle of the lower unit allows the bow to be trimmed up or down when the boat is moving.

Shallow-water operation is possible at slow speeds by tilting the lower unit upward until the propeller begins to break the surface.

The lower unit can be tilted up and out of harm's way when the boat is being trailered over the highway.

Stern drives are not perfect. Power from the engine has to go through a universal joint and then make two right angle bends to reach the propeller. All that twisting and turning add complexity, and complexity means more parts to break or wear out. Even so, there's a long stern drive history and it has proven itself a popular method for propelling powerboats less than 30 feet long.

A major advancement in stern-drive design has been the development of counter-rotating propellers. Two propellers are mounted on the same lower unit of the drive (on the same propeller spindle), with one in front of the other. They rotate in opposite directions.

Originally developed for torpedoes and nuclear submarines, counter-rotating props improve several aspects of performance in certain boats. They help heavier cuddy-cabin runabouts and pocket cruisers come on plane and acceleration, and generally improve tracking and handling. Boat handling, too, becomes easier because there is no tendency for the lower unit to "wander" and propeller torque (see boat handling section) is eliminated. A comparison between single and twin-prop stern drives can be found here: One Boat, Two Drive Options.

A Closer Look at Inboards and V-drives

Conventional inboard and V-drive transmissions are mounted inside the hull. They may be attached directly to the engine via a large fitting called a "bell housing," or they may be remotely located and connected to the engine by a short length of driveshaft called a "jack shaft." Forward, neutral and reverse are usually accomplished by gears, which work in conjunction with an oil-filled clutch.

The driveshaft of a conventional inboard boat goes through the hull in a fitting called the "shaft log." A "stuffing box" attached to this fitting allows free rotation of the shaft but keeps water from leaking into the boat.

Inboards and V-drives are most commonly offered for tournament-style water-ski and wakeboard boats. Center-mounted inboards contribute to ski-boat balance and especially flat wakes. In the V-drive configuration, the aft placement of the motor helps create larger wakes for boarders.

Outboards are a fully contained engine and drive in one package, and have become one of the most popular power systems around on boats up to 35' or so. Clear, quiet, smooth four-stroke outboards have displaced most two-stroke models in today's market, though there are still some direct-injection two-strokes out there. The latest developments in outboards have to do with control systems. Most manufacturers now offer joystick controls which use computer integration to move your boat forward, aft, or even sideways with a flick of your fingertips. You can find out more about these systems from our Outboard Expert.